Unidirectional microphone

ABSTRACT

A unidirectional microphone in which a second order gradient microphone and a wave-type microphone are combined so as to obtain an improved directional microphone with the low frequency components obtained from the second order gradient microphone and the medium and higher frequency components being obtained from the wave-type microphone so as to obtain a composite output signal which has a uniform and narrow directive pattern and a smooth frequency response characteristic over a wide frequency range.

United States Patent Noboru Tsuchiya, Kanagawa-ken; Kazuo lnventors Kanesuke Kishi;

Shimura, Tokyo, Japan Appl. No. 742,378 Filed July 3, 1968 Patented May 25, 1971 Assignee Sony Corporation Tokyo, Japan Priority July 13, 1967 Japan 42/45210 UNIDIRECTIONAL MICROPHONE 6 Claims, 9 Drawing Figs.

U.S. Cl

Int. Field of Search H04r l/20 VEL) H 13,ss1,012

[56] References Cited UNITED STATES PATENTS 2,301,744 11/1942 Olson 179/1 2,939,922 6/1960 Gorike 179/1 3,204,031 8/1965 Gorike et al. 179/1 Primary Examiner-William C. Cooper Assistant Examiner-Douglas W. Olms v Attorneyl-lill, Sherman, Meroni, Gross & Simpson ABSTRACT: A unidirectional microphone in which a second order gradient microphone and a wave-type microphone are combined so as to obtain an improved directional microphone with the low frequency components obtained from the second order gradient microphone and the medium and higher frequency components being obtained from the wave-type microphone so as to obtain a composite output signal which has a uniform and narrow directive pattern and a smooth frequency response characteristic over a wide frequency range.

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saw u or 4 INVENTORS MIA/50K: 165m 71/050120 750CW/Y4 K0 0 SAM/am Y ATTORNEYS UNIDIRECTIONAL MICROPHONE BACKGROUND OF THE INVENTION l. Field of the Invention This invention relates in general to unidirectional microphones, and in particular to a unidirectional microphone which has uniform response and a narrow directive pattern over a wide frequency range.

2. Description of the Prior Art In making films for motion pictures or TV it is desirable to keep the microphone out of the picture so that it will not be seen. When conventional microphones are used, the noise and reflected sound waves in the vicinity of the microphone interfere with the desired audio signal which is being picked up. To abrogate this, directional microphones have been used such as the pressure or wave-type microphone or the gradient type directional microphone. The pressure or wave-type microphone has a high order of directivity and a good frequency response in its low frequency range; however, it has poor directivity and poor frequency response in the medium and high frequency range.- On the other hand, the gradienttype directional microphone has a high order of directivity in its medium and high frequency ranges but does not have good directivity in its low frequency range.

. SUMMARY OF THE INVENTION The present invention comprises a small sized microphone which has a uniform and narrow directivity pattern and a smooth frequency response characteristic over a wide frequency range which is obtained by combining a pressure or wave-type directional microphone and a gradient-type directional microphone. The microphones are combined so that the pressure or wave-type directional microphone obtains very good directivity and frequency response in the low frequency range with a relatively small pressure or wave-type directional microphone and the medium and high frequency response is obtained with a gradient-type directional microphone so that the combined microphone results in a relatively small highly directive microphone which has linear frequency response over the entire audible range.

Other objects, features and advantages of the present invention will be readily apparent from the following detailed description of certain preferred embodiments thereof taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of the improved microphone according to this invention;

FIG. 2 is an'enlarged detailed view of a portion of the pressure or wave-type directional microphone;

FIG. 3 illustrates a modification of the microphone according to this invention;

FIG. 4 is a graph indicating the frequency response of the second order gradient microphone utilized in the embodiment of FIG. 1;

FIG. 5 is a graph indicating the frequency response of the line microphone illustrated in FIG. 1;

FIG. 6 is a graph illustrating the frequency response of the unidirectional microphone of FIG. 1;

FIG. 7 is a graph indicating the band-pass response of the filters of the microphone illustrated in FIG. 1;

FIG. 8 illustrates the directional characteristics of the unidirectional microphone in accordance with the prior art; and

FIG. 9 is a graph illustrating the directional characteristics of the unidirectional microphone of FIG. 1.

improved DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention discloses a small microphone capable of uniform and narrow directivity pattern which has a smooth frequency response over a broad frequency range.

FIG. 1 illustrates a unidirectional microphone which comprises a wave-type or line microphone 2 combined with a gradient microphone 1. Sound waves 5 from a particular direction are desired to be detected by the microphone and the microphone is pointed toward the source of the waves 5. The second order gradient microphone 1 comprises a pair of condenser microphone capsules 3 and 4 which have a unidirectional or figure 8 characteristic along the direction of their directional axis 5 and are separated by a distance D,. The distance D might be 30 to 40 cm., for example, and the capsules 3 and 4 may have the same characteristics. Polarizing voltages are applied to the microphone capsules 3 and 4 from bias sources 6 and 7 through the resistors 8 and 9, respectively. The direct current bias on the microphone capsules 3 and 4 are of opposite polarity. The output of microphone 3 is connected through capacitors 10 and 11 to the gate of a field effect transistor 12. The output of microphone capsule 4 is supplied to the gate of the field effect transistor 12 through the capacitor 11. The capacitors I0 and 11 block DC components in the output of the microphone capsules 3 and 4.

The line microphone 2 is of the wave-type and comprises an acoustic pipe 13 along the directional axis 5 having its forward end 13a closed. A plurality of acoustic holes 14 are mounted along the acoustic pipe 13. The pipe 13 is tapered so that it has a larger diameter at the end away from the end 13a. A condenser microphone capsule 15 is connected to the end of the acoustic pipe 13 away from the end 13a.

As best shown in FIG. 2, the holes 14 of the acoustic pipe 13 are cup shaped and project at approximately right angle from the longitudinal axis of the acoustic pipe 13. Open holes are formed through the bottom portions of the hole members 14 to allow sound to pass into the pipe 13. The holes are formed so as to resonate at about 10 kilohertz and are filled with acoustic insulating material 16 which might be felt or other suitable material.

The microphone capsule 15 is connected to the polarizing voltage 6 through a resistor 20. The gradient microphone l and the line microphone 2 are mounted in a common housing 17 shown in dotted line and are adapted to detect sound waves coming from the direction, as indicated by the arrow 5.

The low frequency components of the audio signal detected by the gradient microphone 1 are combined with medium and higher frequency components of the line microphone 2. To accomplish this, the output of the microphone capsule 15 of the line microphone 2 is connected to the gate of the field effect transistor 19 through the condenser 18. The drains of the field effect transistors 12 and 19 are connected to each other and to a power source 21 which has its other side grounded. The sources of the field effect transistors 12 and 19 are connected to ground through the resistors 22 and 23 and results in a source-follower circuit. The output of the microphone l is removed from the source of the field effect transistor 12 and the output of the microphone 2 is removed from the source of the field effect transistor 19. The source of field effect transistor 12 is connected to a condenser 24 which has its other side connected to a low pass filter 25 comprising inductor L, and capacitor C The source of the field effect transistor 19 is connected to a high pass filter 26 comprising a capacitor C and the inductor L The output of the high pass filter 26 is connected to one end of a primary 27a of a transformer 27. The midpoint of the primary 27a is connected to ground. The other end of the primary 27a is connected to the output of the low-pass filter 25. The cutoff frequencies of filters 25 and 26 are selected so that they are approximately equal.

The secondary winding 27b of the output transformer 27 is connected to output terminals 28a and 28b, respectively, and supplied to a suitable utilization means, (not shown).

The field effect transistors I2 and 19 and the filters 25 and 26 are mounted inside a shielding member 29. The circuit for combining the outputs of the two microphones is designated generally as 30.

as shown in FIG. 1. When the spacing D is =)\/2 the output of 5 the pair of capsules'3 and 4 is equal to the sum and the highest sensitivity is obtained. If the spacing D is ==to nMwhere n is a positive integral number) the sensitivity is rapidly reduced and deep valleys appear in the characteristic curve, as shown to the right in FIG. 4. When an acoustic wave impinges from the side on the microphones 3 and 4 the result shown in the dotted curve labeled 6=90 is obtained. In this case, the outputs of the capsule microphones 3 and 4 offset each other because they are always of equal amplitude and of opposite phase irrespective of the distance D,.

When the acoustic waves come in from the rear of the microphone, the outputs of the capsules 3 and 4 almost vanish because the capsules themselves are unidirectional and have figure 8 characteristics. This is illustrated in the curve labeled O=l80 in dashed line in FIG. 4. As shown by the curve 0 for frequencies that have a wavelength less than D,, a favorable directional characteristic is obtained.

The line amplifier 2 has a frequency response curve, as indicated in FIG. 5. This microphone is equivalent to connecting in parallel a plural number of acoustic impedance pipes of different lengths. As is apparent from FIG. 5, favorable directivity is obtained in the higher frequency range but in the lower frequency range almost no directional characteristic appears. For example, notice that a solid curve of (i=0 for waves coming in with the direction of arrow 5 of FIG. 1 has a substantially greater output than the 0=90 and 0=l80 above the frequency of f The frequency f is equivalent to the full useful length D which is equal to ds or the spacing between each hole times the number of holes a half wavelength of the acoustic pipe 13. Although the directivity falls off at a frequency of f, equivalent to ds=)\/2, by choosing the distance ds between the holes at 30 mm. and below results in fn=8 -l0 KHz. which presents no problem in practical use.

Consequently, it is desirable to determine the values of the cutoff frequency f,,, of the low pass range filter and the high pass range filter 26 so that the frequency ranges of the microphones-l and 2 may be partly overlapped to obtain a favorable directional characteristic.

FIG. 6 illustrates the frequency response of the combined outputs of the microphones I and 2, and it is to be observed that directional characteristic is obtained throughout the complete frequency band.

In FIG. 7 are indicated the passband characteristics of the filters 25 and 26 with the curve 32 illustrating the characteristic of the loss-pass filter 25 and the curve 31 indicating the characteristic of the high pass filter 26. FIG. 6 indicates the frequency response curve when f ,.=l KHz. when using the filters 25 and 26 having the characteristics shown in FIG. 7 and when the distance D, is =350 mm. and the distance D is =450 It is to be noted from FIG. 6 that the curves indicated 0 90 and 0=l 80 have outputs which are much lower than the curve indicated 0=0, and thus the combined microphone has great directivity 'as well as uniform output over the audible band.

Thus, a microphone is provided which has directional characteristics over the low frequency band as well as the high frequency band. Also, since the distance D between the capsules 3 and 4 of the second order gradient microphone I may be around -40 cm., and because the length of the acoustic pipe 13 of the line microphone 2 may be about 50 cm., the whole microphone system is relatively small. The outer diameter of the acoustic pipe 13 of the line microphone 2 may be about 8 mm., and very little space is required for the microphone.

Any sensitivity differences existing between the microphones 1 and 2 may be regulated by means of the polarizing voltages supplied to the microphone capsules 3, 4

and 15, and the voltages applied to the field effect transistors 12 and 19. Also, microphone capsules of the same general characteristics may be chosen for the capsules 3, 4 and 15.

FIG. 8 illustrates the directional characteristics of line microphones according to the prior art plotted in polar coordinates. FIG. 9, on the other hand, illustrates the directional characteristics of the microphone in accordance with the present invention, which has the same length of, say about 60 cm., of the line microphone of the prior art, shown in FIG. 8. In comparing the two graphs, it will be clearly seen that the microphone according to the present invention exhibits 21 markedly higher directional characteristic in the low frequency ranges such as 50 Hz., I00 Hz, 500 Hz., etc. Thus, a substantially improved directional characteristic is obtained with the microphone of the present invention.

FIG. 3 illustrates a modification of the invention in which two capsules 3 and 15 are utilized rather than three capsules such as were used in the modification of FIG. I. In FIG. 3 the same reference numerals are used to identify the same elements in FIGS. 1 and 3. The capsule IS in the modification of FIG. 3 serves as a portion of the gradient microphone and the wave-type microphone. It is to be noted that the output of the capsule is is coupled through the capacitor 18 to the gate of the field effect transistor 19 and through the capacitor 11 to the gate of the field effect transistor 12. Thus, its output is combined with the output of the capsule 3 to form a second order gradient microphone. It is to be noted that the capsule 15 is biased through the resistor 9 from the power source 7, whereas the capsule 3 is biased through the resistor 8 from the power source 6. The bias on the capsules 3 and 15 are of opposite polarity so that a differential output from the second order gradient microphone will be obtained and supplied to the gate of the field effect transistor 12. Thus, the capsules 3 and I5 produce a second order gradient microphone equivalent to the capsules 3 and 4 in FIG. I. The capsule 15 also serves as the output of the wave-type microphone 2 and supplies this output through the capacitor 18 to the gate of the field effect transistor I9.

The combined outputs of the two microphones appear at terminals 28a and 28b and the results are that the unidirectional microphone shown in FIG. 3 exhibits the same characteristics as the unidirectional microphone in FIG. 1.

Although condenser-type microphone capsules may be used for the capsules 3, 4 and 15, it is to be realized that any other type of dynamic or ribbon type of microphone may be utilized.

Although minor modifications might be suggested by those versed in the art, it should be understood that we wish to embody within the scope of the patent warranted hereon all such modifications as reasonably and properly come within the scope of our contribution to the art.

We claim:

I. A unidirectional microphone comprising:

a directional wave-type microphone;

a directional second order gradient microphone mounted adjacent said wave-type microphone and orientated so that both microphones have the same directivity;

combining means connected to both microphones and comprising a high pass filter and a low pass filter with the output of the second order gradient microphone connected to the low pass filter and the output of the wave-type microphone connected to the high pass filter;

output terminals connected to the low and high pass filters;

said second order gradient microphone comprising a pair of microphone elements separated by a distance D and biasing means connected to said pair of microphone elements to provide a bias of opposite polarity on said elements; and wherein said wave-type microphone comprises an acoustic pipe with a closed end and spaced acoustic openings formed along its longitudinal axis and one of said pair of microphone elements mounted at one end of said pipe so that in addition to forming a portion of the gradient microphone pickup it also provides pickup for the wave-type microphone.

2. A unidirectional microphone comprising:

a directional wave-type microphone;

a directional second order gradient microphone mounted adjacent said wave-type microphone and orientated so that both microphones have the same directivity;

combining means connected to both microphones and comprising a high pass filter and a low pass filter with the output of the second order gradient microphone connected to the low pass filter and the output of the wave-type microphone connected to the high pass filter;

output terminals connected to the low and high pass filters;

said wave-type microphone comprising an acoustic pipe with a closed end and spaced acoustic openings formed along its longitudinal axis and a microphone element mounted at one end of said .pipe; and wherein the length of said pipe is D and wherein said second order gradient microphone comprises a pair of microphone elements separated by a distance D,, and D is greater than D 3. A unidirectional microphone according to claim 2 wherein the wave-type microphone comprises an acoustic pipe having numerous spaced acoustic holes formed along its longitudinal direction and having one end closed, and one of said pair of gradient microphone elements linked to the other end of said acoustic pipe.

4. A unidirectional microphone according to claim 2 wherein the combining means comprises a low pass filter which receives the output of the second order gradient microphone, a high pass filter which receives the output of the wave-type microphone, and an output transformer having a primary connected to the low pass and high pass filters, and said output transformer having a secondary across which an output signal is produced.

5. A unidirectional microphone according to claim 2 comprising a first field effect transistor having three electrodes with a first electrode coupled to the output of the second order gradient microphone and the other two electrodes connected in circuit with the low pass filter, a second field effect transistor having three electrodes with a first electrode connected to the output of said wave-type microphone and with its other two electrodes connected in circuit with the high pass filter.

6. A unidirectional microphone comprising:

a second order gradient microphone having a pair of gradient microphone elements spaced apart so that they have approximately equal directivity and producing a composite output signal;

a wave-type microphone mounted so that it has a directivity which substantially coincides with that of said second order gradient microphone;

combining means receiving the outputs of said microphones to combine the low frequency components from said second order gradient microphone with the medium and high frequency components of the wave-type microphone;

said wave-type microphone comprising an acoustic pipe with a closed end and spaced acoustic openings formed along its longitudinal axis and a microphone element mounted at one end of said pipe wherein the length of said pipe is D and wherein said second order gradient microphone comprises a pair of microphone elements separated by a distance D and D is greater than D,. 

1. A unidirectional microphone comprising: a directional wave-type microphone; a directional second order gradient microphone mounted adjacent said wave-type microphone and orientated so that both microphones have the same directivity; combining means connected to both microphones and comprising a high pass filter and a low pass filter with the output of the second order gradient microphone connected to the low pass filter and the output of the wave-type microphone connected to the high pass filter; output terminals connected to the low and high pass filters; said second order gradient microphone comprising a pair of microphone elements separated by a distance D1 and biasing means connected to said pair of microphone elements to provide a bias of opposite polarity on said elements; and wherein said wave-type microphone comprises an acoustic pipe with a closed end and spaced acoustic openings formed along its longitudinal axis and one of said pair of microphone elements mounted at one end of said pipe so that in addition to forming a portion of the gradient microphone pickup it also provides pickup for the wave-type microphone.
 2. A unidirectional microphone comprising: a directional wave-type microphone; a directional second order gradient microphone mounted adjacent said wave-type microphone and orientated so that both microphones have the same directivity; combining means connected to both microphones and comprising a high pass filter and a low pass filter with the output of the second order gradient microphone connected to the low pass filter and the output of the wave-type microphone connected to the high pass filter; output terminals connected to the low and high pass filters; said wave-type microphone comprising an acoustic pipe with a closed end and spaced acoustic openings formed along its longitudinal axis and a microphone element mounted at one end of said pipe; and wherein the length of said pipe is D2 and wherein said second order gradient microphone comprises a pair of microphone elements separated by a distance D1, and D2 is greater than D1.
 3. A unidirectional microphone according to claim 2 wherein the wave-type microphone comprises an acoustic pipe having numerous spaced acoustic holes formed along its longitudinal direction and having one end closed, and one of said pair of gradient microphone elements linked to the other end of said acoustic pipe.
 4. A unidirectional microphone according to claim 2 wherein the combining means comprises a low pass filter which receives the output of the second order gradient microphone, a high pass filter which receives the output of the wave-type microphone, and an output transformer having a primary connected to the low pass and high pass filters, and said output transformer having a secondary across which an output signal is produced.
 5. A unidirectional microphone according to claim 2 comprising a first field effect transistor having three electrodes with a first electrode coupled to the output of the second order gradient microphone and the other two electrodes connected in circuit with the low pass filter, a second field effect transistor having three electrodes with a first electrode connected to the output of said wave-type microphone and with its other two electrodes connected in circuit with the high pass filter.
 6. A unidirectional microphone compRising: a second order gradient microphone having a pair of gradient microphone elements spaced apart so that they have approximately equal directivity and producing a composite output signal; a wave-type microphone mounted so that it has a directivity which substantially coincides with that of said second order gradient microphone; combining means receiving the outputs of said microphones to combine the low frequency components from said second order gradient microphone with the medium and high frequency components of the wave-type microphone; said wave-type microphone comprising an acoustic pipe with a closed end and spaced acoustic openings formed along its longitudinal axis and a microphone element mounted at one end of said pipe wherein the length of said pipe is D2, and wherein said second order gradient microphone comprises a pair of microphone elements separated by a distance D1, and D2 is greater than D1. 